1. Technical Field
The present invention relates to a digital acoustic playback system for direct conversion of a digital signal into analog audio, and in particular relates to a multi-way digital acoustic playback system and applications using the same.
2. Description of Related Art
WO2007/135928A1 discloses a digital/analog conversion apparatus which directly converts a digital signal into analog audio. In WO2007/135928A1, a system which directly converts a digital signal into analog audio by using a circuit which is supplied with a digital audio signal as an input and which outputs a plurality of digital signals and a plurality of coils (units) which are driven by the plurality of digital signals is disclosed. Below, this type of system is referred to as a digital acoustic playback system.
This type of digital acoustic playback system has a characteristic of lower power consumption compared to an analog acoustic playback system which drives a speaker using an analog electrical signal. In addition, because a plurality of speakers or a plurality of driving units (such as coils) are used, a large sound pressure with a low electrical voltage is possible compared to a conventional analog audio system which uses a single speaker or single driving unit.
An acoustic playback system configured with a combination of a plurality of speakers, each covering a different frequency range (a different frequency band) is known. This type of acoustic playback system is referred to as a multi-way acoustic playback system. In this type of acoustic playback system, speakers are driven by generating analog signals for different frequency bands, each of which drives a speaker, by a network circuit which uses an analog LCR filter. Therefore, in order to configure a multi-way acoustic playback system using a digital acoustic playback system, a digital sound signal is converted into an analog signal, and the analog signal is converted into analog signals of respective bandwidths through the network circuit, then the converted analog signals must be converted to the digital signals before being input to the digital acoustic playback system. As a result, the conventional multi-way acoustic playback system has the problem of possessing a complex configuration.
FIG. 1 shows a representative conventional example of a multi-way analog acoustic playback system which uses speaker devices (analog speakers) driven by an analog signal. An analog audio signal (101) is converted to analog driving signals (103) for driving speakers after being power-amplified by an analog amplifier (102) and being divided to a plurality of analog signals (105a˜105n) corresponding to a plurality of frequency bands by a network circuit (104) comprised from an analog LRC filter circuit, and the plurality of analog signals are input to speakers (106a˜106n) covering a plurality of different bandwidths. This type of system is a typical embodiment of a multi-way analog acoustic playback system which uses an analog amplifier and speakers covering a plurality of bands.
A different example of a conventional multi-way analog system which uses analog speakers is shown in FIG. 2. A digital audio signal (201) is divided to a plurality of digital signals (203a˜203n) corresponding to a plurality of frequency bands by a digital filter (202). The plurality of digital signals corresponding to the plurality of frequency bands are converted to a plurality of analog signals (205a˜205n) corresponding to the plurality of frequency bands by a plurality of digital analog converters (204a˜204n). Then, a plurality of analog drive signals (207a˜207n) which are power-amplified by a plurality of analog amplifiers (206a˜206n) drive speakers (208a˜208n) which cover the plurality of different bands. A disadvantage in this conventional example is that a digital analog device and amplifier are required for each of the speakers which cover the plurality of different bands. However, because it is possible to realize a filter by digital signal processing, such systems are used for high quality audio in order to realize various band characteristics. This type of system is a typical embodiment of a so-called by drive type multi-way analog acoustic playback system.
FIG. 3 shows an example of a multi-way digital acoustic playback system configured with a digital acoustic playback system comprised of a circuit which is supplied with a digital audio signal as an input and outputs a plurality of digital signals and a plurality of coils (units) driven by the plurality of digital signals. After a digital audio signal (301) is once converted to an analog signal (303) by a digital analog converter (302), it is divided into analog signals (305a˜305n) corresponding to a plurality of frequency bands by a network circuit (304) comprised of an analog RLC filter circuit. Respective analog signals are converted again to respective digital signals (307a˜307n) corresponding to a plurality of frequency bands by a plurality of analog digital converters (306a˜306n). The digital signals are converted to a plurality of digital signals corresponding to the plurality of frequency bands by digital modulation circuits (308a˜308n) which are supplied with digital audio, signals as inputs and which output a plurality of digital signals and the converted plurality of digital signals are supplied as inputs to speakers (310a˜310n) which have a plurality of coils which cover the plurality of different bands. When this type of multi-way acoustic playback system comprised of an analog circuit is simply combined with the digital acoustic playback system shown for example in WO2007/135928A1 in the way described above, a problem arises that the number of required components becomes large.